The present invention is concerned with brake apparatus for railway vehicles and particularly to such brake apparatus as disclosed in U.S. patent application, Ser. No. 195,149 filed Oct. 8, 1980, now U.S. Pat. No. 4,339,155 assigned to the assignee of the present invention.
In the above-mentioned patent application, there is shown a dual-piston brake cylinder device that is arranged to provide for storage of sufficient air as necessary to develop the desired brake forces, without the need for separate, conventional storage reservoirs. The dual pistons are tandem-connected, one being larger than the other and cooperating with the brake cylinder body to form air storage chambers on its opposite sides.
During brake release, air is supplied to these storage chambers by a control valve device that is operative in response to variations in the train brake pipe pressure. When a brake application is initiated, the control valve device connects air from one side of the larger power piston to the opposite side of the smaller positioning piston. A one-way check valve device is provided to effect pressure equalization between the opposite sides of the larger piston to permit movement of the dual pistons in a brake application direction under the influence of the pressure acting on the smaller piston. Once the tandem-connected pistons are in application position, the control valve releases pressure from the one side of the power piston to establish a pressure differential thereacross and accordingly produce braking force.
In order to conserve pressurized air, it is of primary importance to assure complete movement of the dual piston assembly to brake application position before allowing the air to exhaust from the one side of the larger piston, since in application position, the volume of air in the chamber formed at the one side of the larger piston is minimized. In the aforementioned patent, complete movement of the dual pistion assembly to brake application position is determined by sensing the pressure effective at the positioning piston by means of a transfer valve, which in turn pilots a valve to cause the air in the chamber at the one side of the larger piston to be exhausted, while concurrently terminating further pressurization of the positioning piston. The point at which the transfer valve thus operates to cut-off further pressurization of the positioning piston and to exhaust the chamber on the one side of the power piston is hereinafter referred to as the "transfer point".
It will be appreciated that in the foregoing Patent, the positioning piston pressure at which this "transfer point" occurs is predetermined regardless of the pressure existing in the air storage chambers at the time of a brake application. Since in actual practice, brake rigging friction varies, as do other factors which influence actual movement of the brake rigging, the pressure required to establish complete movement of the dual piston assembly to brake application position also varies. When the preset "transfer point" occurs before complete movement of the piston assembly is realized, pressure is exhausted from the one side of the power piston before the volume of air thereat is reduced to its minimum. Consequently, an excess amount of air is used to effect application of the brakes. This problem could be overcome by designing the transfer valve to actuate at a relatively high positioning piston pressure to assure complete movement of the dual piston assembly to brake application position under the worst contemplated conditions of brake rigging friction etc., before effecting release of pressure from the one side of the power piston.
It will be appreciated, however, that such a high pressure setting of the "transfer point" precludes obtaining a fully effective brake application under conditions of low train pressure, since the equalization of pressures from the one side of the power piston to the opposite side of the positioning piston could fail to establish sufficient pressure to actuate the transfer valve, and therefore the "transfer point" could not be reached. Without actuation of the transfer valve, of course, the pressure differential capable of being developed across the power piston is reduced, thus degrading the brake force development.
It is therefore an object of the present invention to provide a transfer valve having a variable "transfer point" predicated on the level of pressure to which the brake equipment is charged at the time of a brake application.
By providing a high "transfer point" in order to assure completion of the piston stroke before venting the release chamber pressure, however, a situation arises in which a nonlinear brake force is produced, i.e., a force which is not continuously proportional to the amount of brake pipe pressure reduction. During the interim period after the piston movement toward brake application position has stopped and prior to subsequent actuation of the transfer valve device, a brake force is developed in accordance with the degree of pressurization of the positioning chamber, as well as the pressure differential developing across the power piston. Following actuation of the transfer valve device, however, the effective positioning chamber pressure remains fixed and only the developing pressure differential across the power piston changes. Consequently, the resultant brake force follows a different "curve" and is therefore not linear.
It is another object of the present invention, therefore, to provide a brake force buildup curve that is substantially linear, irrespective of the "transfer point" at which the transfer valve device is actuated, thereby assuring compatibility with conventional brake equipments in the brake force realized in response to a given brake pipe reduction.
It is yet another object of the invention to provide an empty/load brake control arrangement for modifying the brake force realized in accordance with the vehicle load condition during both service and emergency brake applications.